Red phosphorus is known to be produced from white phosphorus by thermal conversion on an industrial scale in batch procedures. In one commercial procedure, white phosphorus is heated in a substantially inert atmosphere above the red phosphorus transition temperature in an enclosed container. After completion of the reaction, the mass of red phosphorus product is removed from the container by disintegration.
This commercial procedure has many drawbacks, mainly of an environmental nature, which arise from incomplete conversion of white phosphorus to red phosphorus and the nature of the process. For example, toxic white phosphorus fumes may be emitted on opening the container; removal of the residual white phosphorus from the red phosphorus, typically by addition of soda ash followed by leaching, produces a phosphate solution which requires disposal, and the disintegration of the red phosphorus product for removal from the reactor by chipping leads to dust formation and hazards to operators.
Another commercial procedure involves heating white phosphorus in an inert atmosphere in a ball mill at about 250.degree. C. to convert approximately half of the white phosphorus to red phosphorus. After this initial conversion, the conversion is allowed to proceed to completion at a temperature in excess of about 340.degree. C. No positive temperature control is applied during this conversion and only the heat capacity of the system limits the temperature rise. Operation of the ball mill during the conversion results in the formation of finely divided red phosphorus.
The latter procedure suffers from the disadvantage of lack of control of the extent of heat generation in the second stage, which may lead to an uncontrollable temperature rise. Additionally, equipment and installation costs are very high.
While these commercial batch systems suffer from drawbacks, the drawbacks are tolerated for the lack of a commercially viable alternative. Similarly, batch systems are used for the lack of a commercially viable continuous process.
The process for the production of red phosphorus by the thermal conversion of white phosphorus is an exothermic one. The rate of conversion increases rapidly with increasing temperature above about 300.degree. C., so that the heat evolved may cause an uncontrolled reaction which may ultimately lead to phosphorus vapor explosions at high temperatures. In providing a safe, efficient process for the production of red phosphorus, care must be taken to control the heating of the system caused by the exothermic reaction.